1. Field of the Invention
The present invention is broadly concerned with improved techniques for the adsorption of target compounds through use of finely divided adsorbents selected from the group consisting of MgO, CaO, TiO.sub.2, ZrO.sub.2, Fe.sub.2 O.sub.3, NiO, CuO, Al.sub.2 O.sub.3, ZnO and mixtures thereof. The most preferred adsorbents of the invention are prepared by aerogel methods and have an average particle size of from about 1-20 nm. In the method, target compound(s) are contacted with the adsorbent to destructively adsorb or chemisorb the target compound(s). In particularly preferred forms, the adsorption reaction may be carried out at ambient temperatures and at atmospheric pressures.
2. Description of the Prior Art
In recent years, the scientific community and the public at large have expressed increasing concern about the effect of toxic chemicals on the environment. Spills of liquid chemicals or the inadvertent release of gaseous pollutants can create extreme environmental hazards which must be effectively cleaned up and controlled. To give another example, several million tons of phosphorous nerve agents exist in the United States, and investigations are under way to find safe, effective measures to detoxify these chemicals without endangering human life or the environment.
Technology currently in use includes activated carbon adsorbents (black powders) and/or highly caustic solutions. Activated carbon suffers from the fact that it does not destroy a toxic chemical but merely "holds it" by adsorption forces (i.e., chemisorption). Moreover, inorganic pollutants such as hydrogen cyanide, cyanogen chloride and acid gases are not adsorbed well by activated carbon. Finally, activated carbon is difficult to clean up. Caustic wash solutions create problems because of their tendency to corrode and degrade metals, paint and wood. Moreover, these solutions are inherently heavy and very dangerous to handle.
It has been suggested in the past to employ ultrafine (nanoscale) particles of MgO or CaO for the destructive adsorption of chlorinated benzenes, Li et al., Environmental Science & Technology, 28:1248-1253 (1994). In this paper, high temperature destructive adsorption of chloroarenes was facilitated by the presence of the nanoscale oxides. Similarly, Koper et al., Chem. Mater., 5:500-505 (1993) described the destructive adsorption of chlorinated hydrocarbons at high temperatures using nanoscale CaO. Finally, Klabunde et al., High Temp. Mater. Sci., 33:99-106 (1995) advocates the destructive adsorption of chlorocarbons at high temperature through use of MgO/Fe.sub.2 O.sub.3 composites.
Lin et al. (Langmuir, Vol. 1, No. 5, pp.600-605, 1985) describe the use of MgO and CaO for adsorbing organophosphorous compounds at subatmospheric pressures. The paper also reported destructive adsorption of certain compounds at temperatures of 100.degree. C. and above. There is no teaching in this reference of adsorption at atmospheric pressures or above, nor any suggestion that destructive adsorption can occur at lower temperatures.
Li et al. (Chem. Mater., Vol. 4, No. 2, pp. 323-330, 1992) describes the high temperature destructive adsorption of organophosphorous compounds using nanoparticle MgO and CaO. The lowest effective destructive adsorption temperature described in this reference is 170.degree. C., with best results being achieved at temperatures of 700-900.degree. C.
These prior techniques all generally require relatively high reaction temperatures which consequently limits the utility of the adsorbence reactions. Moreover, there are a large number of potentially troublesome toxic materials such as nerve gases which are not addressed by these prior methods. There is accordingly a need in the art for improved adsorbent methods having broad applicability to a wide variety of target compounds and which can be carried out at relatively low and even room temperatures.